Ink-jet recording method and device

ABSTRACT

An ink-jet recording method which hardly causes ink to blur. To this end, there are provided a correction table ( 30 ) which contains the number of ejection ink dots of each color limited in advance according to the combination of the numbers of ejection ink dots instructed for respective colors in a unit area in such a way that the total number of ejection ink dots does not exceed a predetermined limit value, and an error table ( 31 ) which contains the errors which may be produced when determining the limited numbers of ejection ink dots of the colors and which are previously calculated and determined according to the combination of the numbers of ejection ink dots instructed for the colors in the unit area. Before printing, a corrected number of ejection ink dots for each color in each unit area is determined from the correction table ( 30 ) according to the combination of the numbers of ejection ink dots on which each head is instructed based on binary image data (S 25 ). At this time, the errors listed in the error table ( 31 ) are accumulated for every unit area (S 27 ), the integer part, if appears in the accumulated value, is made to affect the numbers of ejection ink dots, and the accumulated error is updated (S 25 ).

TECHNICAL FIELD

The present invention relates to an ink-jet recording device whichrecords a color image by putting a plurality of ink dots on the samepixel position using multiple-ink-color heads.

BACKGROUND ART

Usually, an ink-jet recording device repeatedly moves a recording head,with a plurality of ink-ejecting nozzles arranged thereon, in thedirection different from the nozzle arrangement direction to form a bandof image area (band) during one movement. This repeated one-band imagerecording, one band for each specific amount of movement of recordingmedium, completes one whole image recording.

Today, there are a variety of recording media for use in the ink-jetrecording device. Among these recording media, print results look muchmore beautiful on lustrous paper or film than on plain paper.Furthermore, the ink-jet recording device may print on cloth such ascotton or enamel. This demand will grow in future.

However, some of these media described above do not absorb ink well, andsome others are easily blurred. Printing on these recording mediaresults in an overflow of ink in a high-density print area,significantly degrading the print image quality. This degradationdepends on the characteristics of the recording media. The overflow ofink also dirties some mechanical parts of the ink-jet recording device,such as a platen, sometimes affecting print operations.

The conventional solution to this problem is that the ink ejectionamount is limited by a masking that is performed during thepreprocessing of conversion from gray-scale images to binary (bi-level)images.

However, the prior art described above has the drawbacks described belowbecause the correction is made on an image signal basis.

First, a popular ink-jet recording device is connected, in most cases,to a computer terminal. This computer terminal performs imageprocessing, such as conversion to binary values, and transfers theprocessed result to the ink-jet recording device for recording thereon.However, an image input unit or an image processing unit which processesmulti-valued signals, if built in an ink-jet recording device, wouldincrease the cost and the processing time and, therefore, they are notstandard on a popular ink-jet recording device. When an ink-jetrecording device performs correction on an image signal basis,conversion from gray-scale images to binary images waists time and therecording speed is reduced. An ink-jet recording device according to thepresent invention makes correction, not on multi-valued signal, but onbinary image data.

It is therefore an object of the present invention to prevent imagequality degradation caused by a blur on the recording medium bylimiting, through binary image processing, the ink ejection amount to alevel at which no image degradation is caused by a blur on the recordingmedium.

It is another object of the present invention to reduce the change incolor and to produce a good-quality color image recording even when theink ejection amount is limited during color image recording.

DISCLOSURE OF INVENTION

An ink-jet recording method according to the present invention is anink-jet recording method for putting a plurality of ink dots on the samepixel position using multiple-color ink heads to record a color image,the ink-jet recording method comprising the steps of:

(a) dividing a whole image area into a plurality of unit areas eachcomposed of a predetermined plurality of pixels;

(b) for each unit area of the image to be recorded, calculating a totalof ejection ink dots instructed to be printed in the unit area;

(c) checking if the calculated total of ink dots exceeds a predeterminedlimit value;

(d) if so, proportionally distributing the limit value to colorsaccording to mutual ratios among numbers of ink dots of the colors to beejected in the unit area and rounding each of distribution results to aninteger to determine a number of ejection ink dots of each color in theunit area;

(e) correcting the instructed number of ink dots of each color byreducing the same based on the determined number of ejection ink dots ofeach color; and

(f) ejecting ink from each head into the unit area according to thecorrected number of ejection ink dots.

As described above, a check is made for each unit area as to whether thetotal of the ink dots therein exceeds a predetermined limit value. Ifthe total exceeds the predetermined limit value, the number of ejectionink dots of each color is reduced in such a way that the limit value isproportionally distributed among the colors according to the ratio ofthe number of ejection ink dots of each color in the unit area. As aresult, the image quality degradation of a printed matter caused by anink blur may be reduced by limiting the ink ejection amount in ahigh-density print area to a given level while keeping the change incolor to a minimum.

Preferably, when reducing the number of ejection ink dots in step (e),the ejection ink dots are thinned out according to an ejection ink dotreduction priority pre-established for each pixel position in the unitarea. More specifically, the priority is established so that theejection ink dots of adjacent pixels in the unit area are not thinnedout consecutively and so that thinning-out orders of adjacent pixels aredifferent between adjacent unit areas. This prevents an unevendistribution in the image caused by thinning-out.

Preferably, an error generated by rounding the result of theproportional distribution of the limit value to the integer, in step(d), is accumulated and passed to an adjacent unit area for each areaand, when an absolute value of the accumulation value becomes 1 orlarger, the accumulation value is reflected on the determined number ofejection ink dots of each color in the adjacent unit area. Thiscompensates for an error generated by rounding and substantiallyprevents the color from being changed.

When the total of the numbers of ejection ink dots of the colors exceedsthe limit value, the number of ejection ink dots of the colors isreduced by the exceeded number of the ejection ink dots according to apredetermined color order, and, for the color whose ejection ink dotshave been reduced, the number of reduced ejection ink dots is added tothe error of the color in the unit area. This prevents the total of theejection ink dots from exceeding the limit value after the numbers ofejection ink dots are rounded to integers.

An ink-jet recording device according to the present invention is anink-jet recording device which puts a plurality of ejection ink dots onthe same pixel position using multiple-color ink heads to record a colorimage, the ink-jet recording device comprising a plurality of heads eachof which ejects ink droplets based on binary image data; a correctiontable which contains a number of ejection ink dots of each head limitedin advance according to a combination of the numbers of ejection inkdots instructed to be ejected for respective colors in a unit area insuch a way that the total number of ejection ink dots does not exceed apredetermined limit value, the unit area being composed of apredetermined plurality of pixels; an error table which contains errorswhich may be produced when determining the limited numbers of ejectionink dots of the colors and which are previously calculated anddetermined according to the combination of the numbers of ejection inkdots specified for the colors in the unit area; and a control means forcontrolling an ink ejection of the plurality of heads based on thecorrected binary image data, wherein the control means sequentiallyprocesses a plurality of consecutive areas, one unit area at a time,during which the correction table and the error table are referencedaccording to the combination of ejection ink dots of colors to beejected in the unit area based on the binary image data and the errorobtained from the error table is passed sequentially to a subsequentunit area for accumulating the error for each color and wherein, for theunit area, a corrected number of ejection ink dots for each head isobtained from the correction table, an integer part of an absolute valueof the accumulated error of a color in the unit area, if 1 or larger, isreflected on the number of the corrected ejection ink dots of the colorin the unit area, the accumulated error is updated according to thereflection, and an ink of each color is ejected in the unit areaaccording to the number of corrected number of ejection ink dots onwhich the integer part was reflected. This gives a device whichimplements the method according to the present invention describedabove.

When the ink of each color is ejected in the unit area according to thecorrected number of ejection ink dots on which the integer part wasreflected and if the number of ink dots instructed to be ejected in theimage data must be increased or decreased, the control means increasesor decreases the number of ink dots according to priority predeterminedaccording to pixel positions within the unit area. This priority ispreferably set so that the ejection ink dots of adjacent pixels in theunit area are not increased or decreased consecutively and so that anincrease order or a decrease order of adjacent pixels are differentbetween adjacent unit areas.

The error may be accumulated in consecutive unit areas horizontally orvertically over a whole image area.

Alternatively, the error occurred in the unit area is divided into twohalves, and one half of the error may be accumulated in a horizontaldirection of consecutive unit areas, and the other half in a verticaldirection of consecutive unit areas, over the all unit areas.

A plurality of correction tables and error tables, each having its ownlimit value and/or unit area size, may be provided, and the controlmeans may selectively use the plurality of correction tables and errortables according to various conditions. The various conditions includethe type of recording medium, paper quality, the ink characteristics,the remaining amount of ink during printing, the temperature, thehumidity, etc. This enables the number of ejection ink dots to becorrected according to various conditions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the hardware configuration of anink-jet recording device in an embodiment of the present invention;

FIG. 2 is a flowchart showing the schematic processing flow of theink-jet recording device of FIG. 1 from input data reception toprinting;

FIG. 3 is a diagram showing a unit area for which the correctionprocessing of ink ejection amounts is performed in the embodiment of thepresent invention;

FIG. 4 is a flowchart showing the creation of a correction table and anerror table in the embodiment of the present invention;

FIG. 5 is a flowchart showing the correction processing using thecorrection table and the error table of FIG. 4;

FIG. 6 is a diagram showing the dot thinning-out priority in the unitarea in the embodiment of the present invention;

FIG. 7 is a diagram showing an example of the correction table in theembodiment of the present invention; and

FIG. 8 is a diagram showing an example of the error table in theembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment according to the present invention will be described indetail with reference to the attached drawings.

FIG. 1 is a schematic block diagram showing the hardware configurationof an ink-jet recording device according to the present invention. Inthe figure, numeral 11 indicates a CPU controlling the overall operationof the device. Numeral 12 indicates a RAM for temporarily storingtherein data and programs. Numeral 13 indicates a ROM for storingtherein programs and data, in a non-volatile manner, the program beingexecuted by the CPU 11 to run the ink-jet recording device. Numeral 14indicates an interface via which the device connects to a computerterminal or to other units and to which binarized data is transferredfrom such an external device (not shown). Numeral 15 indicates an LCDfor displaying operation indications, numeral 16 indicates a keyoperation unit for selecting various set-up items of the ink-jetrecording device, numeral 17 is a print unit for driving ink-jet heads,and numeral 18 indicates a system bus.

The heads in the print unit 17 in the embodiment are of four colors,black (K), cyan (C), magenta (M), and yellow (Y), each provided with 128nozzles and ejecting ink droplets corresponding to the binary imagedata.

FIG. 2 is a process flowchart showing the operation from the time inputdata is received to the time the data is printed.

First, input data is received from external (S21), and the received datais analyzed to prepare one band of print data suitable for recordingwith the ink-jet heads (S22). Next, for each color, a check is made forthe ink ejection amount (number of ejection ink dots) of each unit areain each raster (S23), a correction table 30 is referenced based on thecheck result to obtain the corrected ink ejection amount (ink ejectionamount after correction) (S24), and print data is corrected according tothe corrected ink ejection amount (S25). During this correction, thecorrected ink ejection amount is determined by the obtained correctedink ejection amount and an error value which will be described later,and dot thinning-out is performed based on this corrected ink ejectionamount (Dot addition could be performed for error correction as will bedescribed later). Then, an error table 31 is referenced to obtain anerror value corresponding to the ink ejection amount checked in S23(S26), and this error value is added to an accumulated error value 29(S27). Printing is performed according to the corrected ink ejectionamount (S28).

In this embodiment, the correction of ink ejection amount is made foreach unit area, which will be described later, in each raster. The printoperation is performed for one band composed of a plurality of rasters.To do so, a buffer memory for accommodating one band of data is providedin the buffer memory in RAM 12.

FIG. 3 shows a unit for which the correction of ink ejection amount isto be made in this embodiment. Dividing a print image into unit areas,each composed of four horizontal (head movement direction) pixels×one(paper transport direction) pixel, gives the best result in terms ofcost performance. The rightmost unit area less than 4 dots in length isnot processed. It should be noted that the present invention is notlimited to a unit area composed of four pixels×one pixel but may beapplied to a unit area composed of n pixels×one pixel (where, n is aninteger, for example, between two and eight), one pixel×n pixels, or npixels×m pixels (where, m is an integer, for example, between two andeight).

In this specification, the ink implanting density is represented interms of % and, when one dot of one ink color is put on a one-pixelarea, the ink implanting density is defined as 100%. For example, whenfour dots, each corresponding to one of four colors, are put on the sameone-pixel area, the ink implanting density is 400%.

For a unit area composed of a four-pixel area of four horizontalpixels×one vertical pixel described above, when four dots of ink areejected in any position and in any color within the unit area, the inkimplanting density is 100%. Some examples are given below. When fourdots of the same ink color are ejected, but no dot of other ink colorsis ejected, in the unit area, the ink implanting density is 100%. Wheneach of four ink colors is ejected in four dots in the unit area, theink implanting density is 400%. When three dots of the same ink colorare ejected, but no other ink color is ejected, in the same unit area,the ink implanting density is 75%. When four dots of the same ink colorare ejected, and one dot of some other color is ejected, in the sameunit area, the ink implanting density is 125%. When one dot of each offour ink colors is ejected in the same unit area, the ink implantingdensity is 100%, and when two dots of each of four ink colors areejected in the same unit area, the ink implanting density is 200%.

The method according to the present invention divides a print image intosuch unit areas as described above and, for each of the unit areas,corrects (limits) the ink implanting density. If image quality isdegraded by an ink blur when the 100% of ink implanting density isexceeded on a given recording medium, the ink implanting density in allunit areas should be corrected to an ink implanting density that is 100%or lower.

The minimum change unit (%) of the correction of the ink implantingdensity may be changed according to the unit area size. In thisembodiment, since a print image is divided into areas each composed offour horizontal pixels×one vertical pixel, one dot put in one-pixelarea, which is the minimum change unit, corresponds to 25%. For example,when a print image is divided into areas each composed of fivehorizontal pixels×one vertical pixel, one dot of ink ejectioncorresponds to 20% and therefore the ink implanting density can becorrected in increments of 20%. Similarly, when a print image is dividedinto three horizontal pixels×one vertical pixel, one dot of ink ejectioncorresponds to about 33% and therefore the ink implanting density may becorrected in increments of about 33%. These settings may be chosen on anexperimental basis considering the ink blur characteristics of therecording medium and/or the head characteristics.

A target of the present invention is to limit the maximum ink implantingdensity to any given density. As an example, consider a case in which animage is printed so that all pixel areas, each composed of fourhorizontal pixels×one vertical pixel, in a unit area are each composedof two cyan dots and two yellow dots. The resulting image is green withthe ink implanting density of the unit area being 100%.

When the ink implanting density is to be limited to 75%, the totalnumber of ejection ink dots in the unit area must be decreased to threebecause the total number of pixels in the unit area is four. A problemoccurs that which ink dot is to be deleted from the total of fourejection ink dots. If one cyan dot is simply deleted from every unitarea, all unit areas are each composed of one cyan dot and two yellowdots and the target ink implanting density of 75% is attained. However,because one cyan dot is reduced in every unit area, the resulting imageis yellowish green which will be different from the original color. Tocorrect the ink implanting density to 75% without changing the color,dots must be deleted so that a unit area from which one cyan dot isdeleted and a unit area from which one yellow dot is deleted alternatelyappear.

As a second example, consider a case in which an image is printed sothat all unit areas are each composed of four cyan dots and one magentadot. The resulting image is blue which is close to cyan with the inkimplanting density of all unit areas being 125%.

In this case, when the ink implanting density is to be limited to 100%,the total number of ejection ink dots in the unit area must be decreasedto four because the total number of ejection ink dots in the unit areais five. As in the example described above, it is desirable that a unitarea from which one cyan dot is deleted and a unit area from which onemagenta dot is deleted alternate. However, the ratio of the number ofcyan dots to the number of magenta dots is changed from 4:1 to 3.5:0.5and, as the ratio changes, the color changes. In addition, the twoexamples given above are single-color images and therefore not directlyapplicable to complex natural images.

The method according to the present invention intends to correct the inkejection amount while keeping the change in ink color ratio to aminimum. The method for creating the correction table 30 and the errortable 31 that are used for the correction will now be described withreference to the flowchart in FIG. 4.

First, given ink ejection amounts are set in S41. The given ink ejectionamounts of black, cyan, magenta, and yellow are represented as Ik, Ic,Im, and Iy, respectively. The initial values are 0, 0, 0, and 0,respectively. Since the full image area is divided into unit areas eachcomposed of four horizontal pixels x one vertical pixel in thisembodiment, Ik, Ic, Im, and Iy each may take one of five values, 0, 1,2, 3, and 4. Therefore, if the correction table is created using allpossible combinations of the values, there are 625 (=5×5×5×5)combinations. For each of those combinations whose total of all colorvalues exceeds a limit value, the processing shown in FIG. 4 createscorrection data indicating how many dots of what ink color will be usedand stores the created correction data in the correction table.Therefore, the ink ejection amount of each color that is set in S41 issequentially updated until the table creation is finished in S4C whichwill be described later.

In S42, the total Is of the given ink ejection amounts Ik, Ic, Im, andIy of respective ink colors used in the unit area is calculated. Thevalue obtained by converting the limit ink implanting density into thetotal of ink ejection amounts is defined as a limit value IL. In thisembodiment in which the unit area is composed of four pixels, IL=4 whenthe limit ink implanting density is 100%, IL=5 when it is 125%, and IL=6when it is 150%.

In S43, a check is made if Is>IL, that is, if the total Is of the inkejection amounts that were set in S41 exceeds the limit value IL that isthe limit of ink print amount. If the total does not exceeds the limitvalue, dot deletion (thinning-out) is not necessary. Then, the inkejection amounts are stored in the correction table 30 as corrected inkejection amounts corresponding to the ink ejection amounts Ik, Ic, Im,and Iy (S4L). However, because these values are not used, the storage ofthe values may be omitted. In this case, no error calculation isperformed in S4B. In the subsequent step S4M, zeros are stored in thecorresponding locations in the error table 31 as error values. However,because these values are not used, the storage of the values may beomitted. In S4C, it is checked whether table creation is ended, that is,whether all combinations of ink ejection amounts have been processed. Ifthe processing is not finished, control is returned to S41 to set theink ejection amounts of the next combination.

If Is>IL in S43, then it is determined that dot deletion is necessaryand control is passed to S44 that is the next step.

In S44, the corrected ink ejection amount for each color is calculated.Let Tk′, Tc′, Tm′, and Ty′ be the corrected ink ejection amount ofblack, cyan, magenta, and yellow, respectively. Then, the corrected inkejection amounts are calculated by the following expressions:

Tk′=Ik×IL/Is  (1)

Tc′=Ic×IL/Is  (2)

Tm′=Im×IL/Is  (3)

Ty′=Iy×IL/Is  (4)

These expressions indicate that the limit value IL that is the total ofthe ink ejection amounts of all ink colors are proportionallydistributed according to the ink ejection amount of each color. Thetotal of Tk′, Tc′, Tm′, and Ty′ equals IL.

The values of Tk′, Tc′, Tm′, and Ty′ are not always integers. Therefore,the calculation results of expressions (1)-(4) are rounded to thenearest whole numbers to produce integers. The resulting integers areTk, Tc, Tm, and Ty, respectively.

In S45, the total Ts of the corrected ink ejection amounts of the inkcolors, calculated in S44, is calculated. As a result of the rounding ofTk′, Tc′, Tm′, and Ty′, the total of Tk, Tc, Tm, and Ty may exceed thelimit value IL again. Therefore, in S46, a check is made if Ts>IL, thatis, if Tk+Tc+Tm+Ty=Ts exceeds the limit value IL. If not, current valuesof Tk, Tc, Tm, and Ty are stored in the correction table as correctedink ejection amounts. In S4B that follows, error calculation processingis performed. This error calculation processing will be described later.If the total Ts exceeds IL in S46, the judgment processing from S47 toS4A and the associated processing (S4D-S4K) are repeated, as necessary,until Ts matches IL.

In this embodiment, the deletion ink color order, in which the inkejection amounts are decreased until the total Ts matches the limitvalue IL, is black, cyan, magenta, and yellow (in ascending order ofbrightness or saturation). However, it is to be noted that the presentinvention is not limited to this order.

The new corrected ink ejection amounts Tk, Tc, Tm, and Ty, calculated bycorrecting the corrected ink ejection amount according to Ts, are storedin the correction table 30 as “corrected ink ejection amount” (S4L).

When the corrected ink ejection amounts are decided for each combinationof the ink ejection amounts Ik, Ic, Im, and Iy of the ink colors, thecalculation results of expression (1)-(4) are rounded off. This roundingchanges the ratios among the numbers of ejection dots of four ink colorsand therefore generates errors (quantization errors) affecting thecomposed color. S4B is a step to calculate these error amounts.

Let Ek, Ec, Em, and Ey be errors in the amount of black, cyan, magenta,and yellow ink, respectively. These errors are calculated by thefollowing expressions:

Ek=Tk′−Tk  (5)

Ec=Tc′−Tc  (6)

Em=Tm′−Tm  (7)

 Ey=Ty′−Ty  (8)

The calculated values Ek, Ec, Em, and Ey are stored in the error table31 as “errors” corresponding to the combination of the ink ejectionamounts Ik, Ic, Im, and Iy. It should be noted that Tk, Tc, Tm, and Tyin the above expressions are values corrected in S4D-S4G. This is thereason why an example of error table 31 in FIG. 8 contains an errorvalue whose absolute value exceeds 1.

Next, some calculation examples of values stored in the tables will bedescribed. Assume that the ink ejection amounts of the colors, Ik, Ic,Im, and ly, are given as 1, 2, 1, and 3, respectively. Also assume thatthe limit of the ink implanting density is 100% (four ink dots in a unitarea composed of four pixels×one pixel). In this case, the expressions(1)-(4) give the following results:

Tk′=Ik×IL/Is=1×4/7=0.571

Tc′=Ic×IL/Is=2×4/7=1.143

Tm′=Im×IL/Is=1×4/7=0.571

Ty′=Iy×IL/Is=3×4/7=1.714

The resulting values are rounded, with the result that Tk, Tc, Tm, andTy are 1, 1, 1, and 2, respectively. The total Ts of these values is 5,which is larger than the limit value of 4 by 1. This requires that theink ejection amount be decreased by 1. Because the ink ejection amountTk of the first reduction candidate ink color, black, is 1, the value ofTk is reduced by 1 to 0. Thus, Tk, Tc, Tm, and Ty become 0, 1, 1, and 2,respectively. These corrected ink ejection amounts are stored in thecorrection table 30 in the locations corresponding to the ink ejectionamount values Ik, Ic, Im, and ly that are 1, 2, 1, and 3.

In addition, the expressions (5)-(8) described above give the ink colorerror values as follows:

Ek=Tk′−Tk=0.571−0=0.571

Ec=Tc′−Tc=1.143−1=0.143

Em=Tm′−Tm=0.571−1=−0.429

Ey=Ty′−Ty=1.714−2=−0.286

These error values are stored in the error table 31 in the locationscorresponding to the ink ejection amount values Ik, Ic, Im, and Iy thatare 1, 2, 1, and 3.

The correction table 30 and the error table 31 are created as describedabove. This table creation processing is executed only once for tablecreation. Once the values are stored in a non-volatile memory such as aROM, there is no need to perform the processing thereafter.

FIGS. 7 and 8 show the examples in configuration of the correction table30 and the error table 31. Although all combinations ink ejectionamounts Ik, Ic, Im, and Iy are shown for convenience, the table entriesfor combinations in which the total Is of Ik, Ic, Im, and Iy does notexceed the limit value IL need not be included in the tables.

Next, the correction of the number of ejection dots that is performedusing the correction table 30 and the error table 31 during actualprinting will be described with reference to FIG. 5. This correctionprocessing passes an error occurring in a unit area on a raster to asubsequent unit area on the same raster to distribute the error.Therefore, even when an error occurs in a unit area, this correctionprocessing adjusts the error in a subsequent unit area. That is, evenwhen the ink color ratio in a unit area is changed from the originalratio after dot reduction, the error is compensated for in theneighboring areas. Because the size of a unit area is very small withrespect to the full area size of the image, this error distributionrarely degrades image quality.

During the correction processing shown in FIG. 5, an image to beprinted, which is divided into unit areas, are processed from top tobottom, one raster at a time. Within one raster, the unit areas areprocessed sequentially from left to right. First, at the beginning of araster, the error accumulation values are cleared to 0 in S51. InS52-S59 that follow, one raster is processed.

In S52, the total of the ink ejection amounts (total number of ejectionink dots) in the first unit area in the raster is checked. In S53, acheck is made for the total of ink ejection amounts if it exceeds thelimit value (four in this case). If the total exceeds the limit value,the processing in S54 and the subsequent steps are executed. If thetotal does not exceed the limit value, control is passed to S55.

If the total of ink ejection amounts exceeds the limit value, thecorresponding corrected ink ejection amount is obtained first from thecorrection table 30 in S54. Then, in S55, if the accumulated error valuepassed from the preceding unit area is 1.0 or larger, the corrected inkejection amount and the error accumulation value are modified in S5B.That is, the modification is made using the expressions shown below,where Ih is the corrected ink ejection amount in the correction table30, Ir is the corrected ink ejection amount after modification, Er isthe error accumulation value, En is the error accumulation value whosedecimal part is truncated (that is, an integer part), Ia is amodification allowable value, and Sis the size of the unit area.

Ia=En(when S−Ih≧En)  (9)

Ia=S−Ih(when S−Ih<En)  (10)

Ir=Ih+Ia  (11)

Er←Er−Ia

The integer part En of the error accumulation value Er is not useddirectly to modify (increase) the corrected ink ejection amount Ir inthe correction table 30, but the amount of modification to the correctedink ejection amount Ir is decided according to the value S−Ih which isthe “allowance” of the corrected ink ejection amount Ih with respect tothe unit area size S. Because the maximum number of ink dots in the unitarea is S, the number of ink dots exceeding S−Ih cannot be physicallyincreased. This modification amount is the modification allowable valueIa calculated in expressions (9) and (10).

Some examples are given below. If the error accumulation value Er=1.5,then En=1.0. If Ih=3, then S−Ih=4−3=1≧En=1, then Ia=En=1. Therefore,

Ia=En=1

Er=Er−Ia=1.5−1=0.5

Also, if the error accumulation value Er=2.5, then En=2.0. If Ih=3, thenS−Ih=4−3=1<En=2. Therefore,

Ia=S−Ih=4−3=1

Er=Er−Ia=2.5−1=1.5

If the error accumulation value Er of an ink is equal to or less than−1.0 in S55, the corrected ink ejection amount and the erroraccumulation value are modified according to the following expressionsin S5C:

Ia=−En(when Ih≧−En)  (12)

Ia=Ih(when Ih<−En)  (13)

Ir=Ih−Ia  (14)

Er←Er+Ia

Again,the integer part −En of the error accumulation value Er is notused directly to modify the corrected ink ejection amount Ir in thecorrection table 30, but the amount of modification to the corrected inkejection amount Ir is decided according to the relation of the correctedink ejection amount Ih to −En. The modification amount is decided inthis manner in order to prevent the modified Ih from becoming negative.

The modification to the corrected ink ejection amount and the erroraccumulation value described above is made for each ink color. Even ifthe total of modified ink ejection amounts of all colors exceeds thelimit of the ink ejection amount, the modification is allowed tosuppress the change in color.

In S57, the ejection amount of ink colors in the unit area is correctedbased on the final correction values of ink colors obtained as describedabove.

FIG. 6 shows the priority to be used in correcting the ink ejectionamount in a unit area. This priority is established basically so thatthe ink dots of adjacent pixels in the unit area are not thinned outregardless of the size and the shape of the unit area and so that thepixel thinning-out orders of adjacent unit areas are not the same. Thisarrangement prevents an image quality degradation that may be caused bystripes or the like generated by dot thinning-out.

For example, assume that the ink ejection amount of an ink color isthree and that the corrected ink ejection amount is two. In aneven-numbered raster, a check is made for the data of the leftmost pixel(first pixel) in the unit area. If that pixel data is to be printed, itis determined that the dot is not printed. If the data of the pixel isnot to be printed, the check is made in order of the second pixel fromthe right (third pixel), second pixel from the left (second pixel), andrightmost pixel (fourth pixel). The number of ink dots is reduced untilthe ink ejection amount in the unit area becomes 2 that is the correctedink ejection amount. In an odd-numbered raster, the check is made inorder of the third pixel, second pixel, fourth pixel, and first pixel.

It should be noted that the above principle does not require the dotthinning-out priority be the one shown in the figure. Also, the prioritymay be established similarly for a unit area which is not one pixel×fourpixels in size.

Furthermore, as a result of error accumulation, there is a possibilitythat ink dots are not thinned out but added during the ink ejectionamount correction. In such a case, the addition priority may be the oneshown in FIG. 6.

In S58 that follows S57, the error in the error table 31 correspondingto the ejection amount checked in S52 is obtained and this error isadded to the error accumulation value Er for each ink color.

Then, the correction processing for a given unit area is finished. Next,processing proceeds to the unit area that is right to the unit area thathas been processed. If the next right unit area is at the end of theraster and smaller than four horizontal pixels×one vertical pixel, it isnot processed in this embodiment. The completion of processing of theunit area in the rightmost position of the raster means that processingfor one raster is finished and, as a result of the checking in S59,control is passed to S5A. In S5A, control is passed back to S51 toprocess the next raster until all rasters are processed. When allrasters are processed, the correction processing for the image to beprinted is finished.

When printing is performed by ejecting ink from the heads, one band at atime, the correction processing and the printing processing, shown inFIG. 5, may be repeated for each band.

The processing described above limits the ink ejection amount to apredetermined density to suppress an ink blur and to give a good imagewhile keeping the change in color to a minimum even for a complex imagesuch as a natural picture. In addition, the size of a unit area or thelimit of the ink implanting density may be changed according to theconditions such as ink blur characteristics of the recording medium,remaining amount of ink during printing, temperature, humidity, ordifferences in individual ink-jet recording devices. To meet theserequirements, a plurality of correction tables and error tables,described above, may be prepared for switching the tables to useaccording to the conditions.

Although the error is distributed only in one direction (head movementdirection) in the above embodiment, the error may be divided into twohalves and distributed in two directions, one in the head movementdirection and the other in the direction vertical to the head movement.

Furthermore, in order to reduce the load during correction processingshown in FIG. 5, control may be passed to S59 regardless of the erroraccumulation amount to omit the correction of ink ejection amount if thetotal Ts of ejection amounts does not exceed the limit value in S53.

Industrial Applicability

The present invention, applicable to the design and manufacturing of anink-jet recording device, can prevent image quality degradation causedby an ink blur in a high-density print area where data is printedaccording to the ink implanting density on the recording medium whilekeeping the change in color of a color image to a minimum.

What is claimed is:
 1. An ink-jet recording method for putting aplurality of ink dots on the same pixel position using multiple-colorink heads to record a color image, said ink-jet recording methodcomprising the steps of: (a) dividing a whole image area into aplurality of unit areas each composed of a predetermined plurality ofpixels; (b) for each unit area of the image to be recorded, calculatinga total of ejection ink dots instructed to be printed in the unit area;(c) checking if the calculated total of ink dots exceeds a predeterminedlimit value; (d) if so, proportionally distributing the limit value tocolors according to mutual ratios among numbers of ink dots of thecolors to be ejected in the unit area and rounding each of distributionresults to an integer to determine a number of ejection ink dots of eachcolor in the unit area; (e) correcting the instructed number of ink dotsof each color by reducing the same based on the determined number ofejection ink dots of each color; (f) ejecting ink from each head intothe unit area according to the corrected number of ejection ink dots;and (g) when reducing the number of ejection ink dots in step (e), theejection ink dots are thinned out according to an ejection ink dotreduction priority pre-established for each pixel position in the unitarea.
 2. The ink-jet recording method according to claim 1, wherein thepriority is established so that the ejection ink dots of adjacent pixelsin the unit area are not thinned out consecutively and so thatthinning-out orders of adjacent pixels are different between adjacentunit areas.
 3. An ink-jet recording method for putting a plurality ofink dots on the same pixel position using multiple-color ink heads torecord a color image, said ink-jet recording method comprising the stepsof: (a) dividing a whole image area into a plurality of unit areas eachcomposed of a predetermined plurality of pixels; (b) for each unit areaof the image to be recorded, calculating a total of ejection ink dotsinstructed to be printed in the unit area; (c) checking if thecalculated total of ink dots exceeds a predetermined limit value; (d) ifso, proportionally distributing the limit value to colors according tomutual ratios among numbers of ink dots of the colors to be ejected inthe unit area and rounding each of distribution results to an integer todetermine a number of ejection ink dots of each color in the unit area;(e) correcting the instructed number of ink dots of each color byreducing the same based on the determined number of ejection ink dots ofeach color; and (f) ejecting ink from each head into the unit areaaccording to the corrected number of ejection ink dots; (g) wherein anerror generated by rounding the result of the proportional distributionof the limit value to the integer, in step (d), is accumulated andpassed to an adjacent unit area for each area and, when an absolutevalue of the accumulation value becomes 1 or larger, the accumulationvalue is reflected on the determined number of ejection ink dots of eachcolor in the adjacent unit area.
 4. The ink-jet recording methodaccording to claim 3 wherein, when the total of the numbers of ejectionink dots of the colors exceeds the limit value, the number of ejectionink dots is reduced by the exceeded number of the ejection ink dotsaccording to a predetermined color order, each of said numbers ofejection ink dots of each color having been rounded to an integer, andwherein, for the color whose ejection ink dots have been reduced, thenumber of reduced ejection ink dots is added to the error of the colorin the unit area.
 5. An ink-jet recording device which puts a pluralityof ejection ink dots on the same pixel position using multiple-color inkheads to record a color image, said ink-jet recording device comprising:a plurality of heads each of which ejects ink droplets based on binaryimage data; a correction table which contains a number of ejection inkdots of each head limited in advance according to a combination of thenumbers of ejection ink dots instructed to be ejected for respectivecolors in a unit area in such a way that the total number of ejectionink dots does not exceed a predetermined limit value, said unit areabeing composed of a predetermined plurality of pixels; an error tablewhich contains errors which may be produced when determining the limitednumbers of ejection ink dots of the colors and which are previouslycalculated and determined according to the combination of the numbers ofejection ink dots specified for the colors in the unit area; and acontrol means for controlling an ink ejection of the plurality of headsbased on the corrected binary image data, wherein said control meanssequentially processes a plurality of the consecutive unit areas, oneunit area at a time, during which said correction table and said errortable are referenced according to the combination of ejection ink dotsof colors to be ejected in the unit area based on the binary image dataand the error obtained from said error table is passed sequentially to asubsequent unit area for accumulating the error for each color, andwherein, for the unit area, a corrected number of ejection ink dots foreach head is obtained from said correction table, an integer part of anabsolute value of the accumulated error of a color in the unit area, if1 or larger, is reflected on the number of the corrected ejection inkdots of the color in the unit area, the accumulated error is updatedaccording to the reflection, and an ink of each color is ejected in theunit area according to the number of corrected number of ejection inkdots on which the integer part was reflected.
 6. The ink-jet recordingdevice according to claim 5 wherein when the ink of each color isejected in the unit area according to the corrected number of ejectionink dots on which the integer part was reflected and if the number ofink dots instructed to be ejected in the image data must be increased ordecreased, said control means increases or decreases the number of inkdots according to priority predetermined according to pixel positionswithin the unit area.
 7. The ink-jet recording device according to claim6 wherein the priority is set so that the ejection ink dots of adjacentpixels in the unit area are not increased or decreased consecutively andso that an increase order or a decrease order of adjacent pixels aredifferent between adjacent unit areas.
 8. The ink-jet recording deviceaccording to claim 7 wherein a plurality of correction tables and errortables, each having its own limit value and/or unit area size, areprovided and wherein said control means selectively uses said pluralityof correction tables and error tables according to various conditions.9. The ink-jet recording device according to claim 6 wherein a pluralityof correction tables and error tables, each having its own limit valueand/or unit area size, are provided and wherein said control meansselectively uses said plurality of correction tables and error tablesaccording to various conditions.
 10. The ink-jet recording deviceaccording to claim 5 wherein the error is accumulated in consecutiveunit areas horizontally or vertically over a whole image area.
 11. Theink-jet recording device according to claim 10 wherein a plurality ofcorrection tables and error tables, each having its own limit valueand/or unit area size, are provided and wherein said control meansselectively uses said plurality of correction tables and error tablesaccording to various conditions.
 12. The ink-jet recording deviceaccording to claim 5 wherein the error occurred in the unit area isdivided into two halves and wherein one half of the error is accumulatedin a horizontal direction of consecutive unit areas, and the other halfin a vertical direction of consecutive unit areas, over the all unitareas.
 13. The ink-jet recording device according to claim 12 wherein aplurality of correction tables and error tables, each having its ownlimit value and/or unit area size, are provided and wherein said controlmeans selectively uses said plurality of correction tables and errortables according to various conditions.
 14. The ink-jet recording deviceaccording to claim 5 wherein a plurality of correction tables and errortables, each having its own limit value and/or unit area size, areprovided and wherein said control means selectively uses said pluralityof correction tables and error tables according to various conditions.